In the calibration of sighting devices for guns it is necessary that the axis of a barrel of the gun be parallel to the axis of the optical sighting system or, in some cases, that the axis of the optical sighting system cross the axis of the barrel at a given distance. In almost all large caliber guns, such as those mounted to a tank, the optical sighting system is displaced from the axis of the barrel of the gun. A predetermined relationship of the barrel axis and optical axis must be established and from time to time recalibration must be made. To initially calibrate, or to recalibrate, the relationship of the two aforementioned axes must be determined and a sight made preferably to a calibration target at a given distance. In order for such calibration or recalibration to be accurate and effective, a line of sight along the axis of the barrel must be established. Then the axis of the optical system may be adjusted in accordance with the desired calibration.
Boresighting apparatus for large caliber guns are disclosed in U.S. Pat. No. 1,994,177. Sight adjusters for smaller caliber guns are disclosed in U.S. Pat. Nos. 2,353,272; 2,476,981; 3,228,108; and 4,090,305. The last mentioned patent discloses a rifle sight adjuster where a conical muzzle end adapter engages a end of the muzzle and a member extends therefrom into a barrel and carries a compressable washer type member, which when compressed in the axial direction, will expand radially and theoretically engage the interior periphery of the barrel and center the device therein. A device such as this theoretically could provide centering of the adapter device within the barrel of a gun. However, the accuracy of this device is dependent on the distance the expandable washer is extended into the barrel and the accuracy is subject to wear and abrasion of the expandable washer. Also, the device of this patent requires very accurate positioning and spacing of the expandable washer member on the rod extending within the barrel.
The present invention provides a new and improved boresight adapter in which an accurate centering device may be inserted into the muzzle end of a barrel only two to three diameters of the bore of the barrel and will accurately determine the center of the barrel with very high repeatability.